Human malignant and normal cell lines will be studied for their requirement of L-methionine, for their capability to use D-methionine and D- and L-homocysteine instead of L-methionine, and for their viability under the influence of an L-methionine degrading enzyme. Experimental mouse and rat tumors will be studied under nutritional and enzymatic methionine deprivation, the physiological disposition of the L-methionine degrading enzyme in tumors and host its effect upon the blood levels of all the amino acids. The methionine degrading enzyme will be further studied for physical, physicochemical and chemical properties.